1. Field of the Invention
This invention relates to toothed power transmission belts and, more particularly, to a canvas layer for placement over the belt teeth to enhance resistance of the belt teeth to heat, abrasion, and cracking.
2. Background Art
Transmission belts with longitudinally spaced drive teeth are well known in the prior art. It is known to cover the teeth with a canvas layer to lengthen the belt life. It is known, for example, to use a twill weave canvas layer having the weft yarns thereof made of a woolie finish yarn formed by twisting filaments of nylon-6 or nylon-6,6. It is also known to form the weft yarns by twisting the above nylon-6 or nylon-6,6 filaments with a urethane elastic yarn. It is also known to make the warp yarns from a filament yarn of nylon-6 or nylon-6,6.
Increased demands on power transmission belts, particularly in the automotive industry, have brought to light several of the deficiencies of the conventional canvas layers, as those described above. The trend in the automobile industry has been to use a single belt to drive a large number of accessories, resulting frequently in very high belt loading. This loading, coupled with the fact that the engine compartments within which the belts operate are often at very high temperatures, frequently results in premature failure of the belts. Designers of such belts have striven to prolong the life of toothed belts in such environments.
In response to this demand for longer lasting belts in these high temperature environments, such as in engine compartments, a number of different rubber materials have been used which are durable and long lasting in high temperature environments. Among these preferred rubber compositions are: chlorosulfonated polyethylene rubber (CSM); and hydrogenated acrylonitrile-butadiene rubber (hydrogenated NBR). These rubber compositions have replaced chloroprene rubber (CR), commonly used in the backing layer and/or teeth of conventional transmission belts.
To further improve operating characteristics of toothed power transmission belts, tension members made with glass cords, processed to be embedded in CSM or hydrogenated NBR, have been used which exhibit superior bending and wear resistance at high temperatures to conventional glass cords, which are typically subjected to an adhesion process for embedding in chloroprene rubber.
Another area of focus for belt designers has been the canvas covering layer for the belt teeth, which is a critical component of such belts. The covering layer is not only exposed to the high temperatures in the engine compartment but also itself generates additional heat through friction between the belt and cooperating pulleys as the belt engages with, separates from, and slips lengthwise relative to, cooperating pulleys. To date, in spite of efforts made by belt designers, the cover canvas layers currently employed remain a problem area that commonly leads to premature belt failure. Premature failure is particularly a problem in the high temperature environments, such as in engine compartments.
Commonly, the prior art cover canvas layers are made from fabric made from one of nylon-6 or nylon-6,6. Certain improvements have been made to the treating agents therefor. Nonetheless, teeth with this type of cover layer commonly break off within a short period under high load, high temperature conditions, due primarily to failure of the nylon canvas layer covering the belt base and teeth.
One proposed solution to the tooth severance problem caused by failure of the canvas layer is the use of a cover fabric layer made by twisting a yarn such as one made from an aliphatic polyamide fiber or polyester fiber with a urethane elastic yarn. The difficulty with the above canvas layers is that they adhere very poorly to the belt teeth due primarily to the fact that the canvas layer does not absorb the rubber glue which is used to effect the adhesion. After the belt repeatedly contacts the pulleys, the rubber glue separates, resulting in excessive wear on the belts and/or reduced life.
A still further prior art canvas cover layer is shown in Japanese Utility Model Laid Open No. 62-183147. This belt has a cover canvas with the weft yarns thereof being made up of at least a woolie processed yarn of an aromatic group polyamide fiber and a urethane elastic yarn extending in the lengthwise direction of the belt. The difficulty with the woolie processed yarn of the aromatic polyamide fiber is that the fiber is a polymer having no melting point. Accordingly, the fibers cannot be heated to facilitate their bending and uniform pre-twisting thereof. The result of this is that the yarns may have a non-uniform diameter. Additionally, the above fibers are highly rigid and, when woven, produce an undesirably rough surface, having a texture similar to that of a towel.
In addition to the uneven surface resulting from the use of the above fibers, a toothed belt using the above described cover canvas layer has a random PLD value. PLD is a measure of the distance between the pitch line or center line of the load carrying cords and the bottom of the belt teeth. The result is that the belts produced from an identical mold may have different lengths, which obviously is undesirable. Further, with belts having random PLD values, the teeth interfere with the side walls of cooperating pulley grooves during operation. Still further, there is a stress concentration at the base of the teeth which may result in crack generation at an early stage in the belt life.